Die clamping apparatus and method, and method for adjustment to die thickness using said apparatus

ABSTRACT

In a die clamping apparatus for an injection molding machine or the like, die clamping means for coupling and pressing a moving die plate  9  to a fixed die plate via a tie bar  15  has a die clamping cylinder consisting of two stages of a large-diameter cylinder  11  and a small-diameter cylinder  31  and a ram formed into two stages of a large-diameter ram  13  and a small-diameter ram  25 . The large-diameter ram  13  is inserted in the large-diameter cylinder  11,  and the small-diameter ram  25  is inserted in the small-diameter cylinder  31 . The proximal end portion of the tie bar  15  is connected directly to the small-diameter ram  25.  Large oil chambers  11   c  and  11   d  are provided on both sides of the large-diameter ram  13,  and a small oil chamber  11   e  is provided on the tie bar connection side of the small-diameter ram  25.  An oil pressure is supplied appropriately to each of the large oil chambers and the small oil chamber in the die clamping cylinder to appropriately move the tie bar  15.    
     The apparatus constructed as described above can be manufactured at a low cost, and useless time for eliminating a gap between a ring grooves in the tie bar and an engagement teeth of half nuts caused by the adjustment to die thickness can be reduced.

TECHNICAL FIELD

[0001] The present invention relates to a die clamping apparatus, a dieclamping method, and a method for adjustment to die thickness using thedie clamping apparatus, which are used for an injection molding machineor the like.

BACKGROUND ART

[0002] Conventionally, there is available a die clamping apparatus foran injection molding machine, which includes a fixed die plate forholding a fixed die, a moving die plate for holding a moving die,displacing means for advancing and retreating the moving die plate withrespect to the fixed die plate, and die clamping means for coupling andpressing the moving die plate to the fixed die plate via a tie bar.

[0003] In the die clamping apparatus of this type, when a moltenmaterial is injected, a high die clamping force is required in order torestrain the opening of die due to the pressure in the die, and on theother hand, when a product is taken out, it is required to open the dieby moving it rapidly through a necessary distance.

[0004] Therefore, the conventional die clamping apparatus is configuredso that the moving die plate is advanced or retreated with respect tothe fixed die plate by using a hydraulic cylinder with a small diameterand a long stroke, whereby the die can be moved rapidly when a productis taken out.

[0005] Also, conventionally, there has been used a system in which a dieclamping cylinder with a large diameter and a short stroke is providedto derive a high die clamping force during the die clamping operation,and an oil pressure is applied to a large-diameter ram in the dieclamping cylinder, whereby the moving die plate is coupled and pressedto the fixed die plate via the tie bar with a high die clamping force.

[0006] In the die clamping apparatus as described above, when the die isreplaced by another die with a different thickness, the position of themoving die plate at the time of die closure and opening changes, andaccordingly the connecting position of the moving die plate and the tiebar also changes. Therefore, each time the die is replaced, it isnecessary to change the position of the moving die plate to adjust theconnecting position of the moving die plate and the tie bar.

[0007] Thereupon, various methods for adjustment to die thickness havebeen proposed in which the connecting position of the moving die plateand the tie bar can be adjusted in response to the replacement of die bya die with a different thickness.

[0008] Hereunder, as an example of the adjusting method, the case of adie clamping apparatus described in Japanese Patent ProvisionalPublication No. 8-276482 (No. 276482/1996) will be explained.

[0009]FIG. 4 shows a die clamping apparatus for an injection moldingmachine described in the aforementioned Publication and, moreparticularly, a typical die clamping apparatus provided with a devicefor adjusting the position of a moving die plate.

[0010] This die clamping apparatus has means for coupling and pressing amoving die plate 9 to a fixed die plate 5 via a plurality of tie bars15. This coupling/pressing means includes die clamping cylinders 11incorporated in the fixed die plate 5, the tie bars 15 each of which isconnected directly to a ram 13 in the die clamping cylinder 11 and hasring grooves 15 a at the distal end portion thereof, and half nuts 29installed on the moving die plate 9. The half nuts 29 can position themoving die plate 9 by being engaged with the ring grooves 15 a in thetie bar 15.

[0011] Further, this die clamping apparatus includes driving means (anauxiliary cylinder 47, a piston 49) for advancing and retreating the ram13 when each of the die clamping cylinders 11 is not under pressure(under atmospheric pressure), tie bar position detecting means (aposition sensor 55, a scale 57) for detecting an individual position inthe movement direction of each of the tie bars 15, and moving die plateposition detecting means (a position sensor 41, a scale 43) fordetecting a position in the movement direction of the moving die plate9. The relative position of the half nuts 29 and the tie bar 15 iscalculated individually based on the detection signal sent form thesedetecting means, and thus the aforementioned driving means (not shown)is controlled so that the half nuts 29 are engaged with the ring grooves15 a in the tie bar 15. This control unit uses ordinary means, andtherefore the description thereof is omitted.

DISCLOSURE OF THE INVENTION

[0012] In the device for adjustment to die thickness in the conventionaldie clamping apparatus, as shown in FIG. 4, since many constructionparts are installed on the outside of the fixed tie plate 5, themanufacturing cost is high. Also, before the pressure is increased toclamp the die, it is necessary to move the tie bars 15 to the right inFIG. 4 in advance by sending a pressure oil into the die clampingcylinder 11 in order to eliminate a gap between the ring grooves 15 a inthe die bar 15 and the ring grooves in the half nuts 29. However, thedie clamping cylinder 11 of the die clamping apparatus is generallyformed so as to have a large diameter because it must provide a highclamping force, so that it is necessary to supply a large quantity ofpressure oil to operate the die clamping cylinder 11. Therefore, therearises a problem in that useless time is produced during the time fromwhen the pressure oil is poured to when the tie bar moves.

[0013] Accordingly, an object of the present invention is to provide adie clamping apparatus in which the number of construction parts issmall, so that the apparatus can be manufactured at a low cost, and alsoto provide a die clamping apparatus, a die clamping method, and a methodfor adjustment to die thickness using the apparatus, in which uselesstime for eliminating a gap between the ring grooves in the tie bar andthe engagement teeth of the half nuts can be reduced a loss of time inthe adjustment to die thickness necessary when the die is replaced.

[0014] To achieve the above object, the present invention provides a dieclamping apparatus comprising a fixed die plate for holding a fixed die;a moving die plate for holding a moving die; displacing means foradvancing and retreating the moving die plate with respect to the fixeddie plate; and die clamping means, which is incorporated in either oneof the fixed die plate and the moving die plate, for coupling andpressing the moving die plate to the fixed die plate via a tie bar,wherein the die clamping means is provided with a die clamping cylinderconsisting of two stages of a large-diameter cylinder and asmall-diameter cylinder, and a ram formed into two stages of alarge-diameter ram and a small-diameter ram; the large-diameter ram isinserted in the large-diameter cylinder, and the small-diameter ram isinserted in the small-diameter cylinder; the proximal end portion of thetie bar is connected directly to the small-diameter ram; a large oilchamber is provided on each side of the large-diameter ram, and a smalloil chamber is provided on the tie bar connection side of thesmall-diameter ram; and oil pressure supply means is provided whichappropriately supplies an oil pressure to the large oil chamber and thesmall oil chamber in the die clamping cylinder to appropriately move thetie bar.

[0015] Also, the present invention provides a die clamping method inwhich using the above-described die clamping apparatus, a pressure oilis supplied to the small oil chamber to move the tie bar, and after anengagement gap between the half nuts and the tie bar is eliminated, apressure oil is supplied to the small oil chamber and the large oilchamber on the small ram side to create a die clamping force.

[0016] Further, the present invention provides a method for adjustmentto die thickness, in which when adjustment to die thickness is madeusing the above-described die clamping apparatus, the ram is located ata position of limit end to which the ram can be moved, and after thepressures in the large oil chambers on both sides of the large-diameterram are decreased, a pressure oil is supplied to the small oil chamberto move the ram in the die clamping direction, by which the tie bar islocated at a position at which the ring grooves in the tie bar engageproperly with the ring grooves in the half nuts.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a partially sectioned side view showing one embodimentof an injection molding machine provided with a die clamping apparatusin accordance with the present invention;

[0018]FIG. 2 is a view showing means for detecting positions of a movingdie plate and a tie bar shown in FIG. 1 by an enlarged cross section,and also showing a hydraulic system for supplying a oil pressure to adie clamping cylinder provided in a fixed die plate;

[0019]FIG. 3 is a block diagram showing a die clamping process in theinjection molding machine shown in FIG. 1; and

[0020]FIG. 4 is partially sectioned side view showing one example of aconventional die clamping apparatus.

BEST MODE FOR CARRYING OUT THE INVENTION

[0021] An embodiment of a die clamping apparatus in accordance with thepresent invention will be described with reference to the accompanyingdrawings.

[0022]FIGS. 1 and 2 show an injection molding machine provided with adie clamping apparatus of this embodiment. For convenience ofexplanation, in FIGS. 1 and 2, the same reference numerals are appliedto elements which are the same as those shown in FIG. 4.

[0023] The injection molding machine of this embodiment has a supportframe 1, and a fixed die plate 5 for installing a fixed die 3 is fixedlyprovided on the support frame 1 (see FIG. 1). Also, a moving die plate 9for installing a moving die 7 is provided on the support frame 1 so asto face the fixed die plate 5. Further, rails 39 are fixedly provided onthe support frame 1 so that linear bearings 35 are guided by the rails39. The linear bearings 35 support the moving die plate 9 via a base 37.Thereby, the moving die plate 9 can be moved on the support frame 1along the rails 39.

[0024] The fixed die plate 5 is provided with a plurality of hydraulicdie clamping cylinders (four in the example, but needless to say, thenumber of cylinders is not limited to this number). Each of the dieclamping cylinders is formed into two stages of a large-diametercylinder 11 with a short stroke and a small-diameter cylinder 31. Inresponse to this, a ram inserted in the die clamping cylinder is formedinto two stages of a large-diameter ram 13 and a small-diameter ram 25.The large-diameter ram 13 slidingly moves in the large-diameter cylinder11, and the small-diameter ram 25 slidingly moves in the small-diametercylinder 31. To the center of the small-diameter ram 25, the proximalend portion of a tie bar 15 is directly connected. On the other hand,the moving die plate 9 is formed with insertion holes 27 for the tiebars 15. Therefore, when the moving die plate 9 approaches the fixingdie plate 5 for the purpose of die closure, the tie bar 15 can penetratethe insertion hole 27. In the distal end portion of the tie bar 15, aplurality of ring grooves (or a thread groove) 15 a are formed.

[0025] On both side of the large-diameter ram 13 in the large-diametercylinder 11, large-diameter oil chambers (hereinafter referred to aslarge oil chambers) 11 c and 11 d are formed. Also, on the left-handside in FIG. 2 of the small-diameter ram 25 in the small-diametercylinder 31, a small-diameter oil chamber (hereinafter referred to as asmall oil chamber) 11 e is formed. An opening on the right-hand side ofthe large-diameter cylinder 11 is closed tightly by a cover 45.

[0026] The tie bar 15 is provided with a position sensor 55, and thefixed die plate 5 is provided with a scale 57 having a detection scalegroove or a detection scale magnet. By the position sensor 55 and thescale 57, die bar position detecting means which detects the positionsof the large-diameter ram 13 and the tie bar 15 and the travel distanceof the tie bar 15 is formed.

[0027] Also, the moving die plate 9 is provided with a position sensor41, and a scale 43 having a detection scale groove or a detection scalemagnet is fixedly provided on a member installed to the support frame 1.By the position sensor 41 and the scale 43, moving die plate positiondetecting means which detects the position and travel distance of themoving die plate 9 is formed.

[0028] At one side of the fixed die plate 5 is fixedly provided ahydraulic moving cylinder 17. The proximal end portion of a rod 23 isconnected directly to a piston 21 in the moving cylinder 17, and thedistal end of the rod 23 is fixed to the side face of the moving dieplate 9.

[0029] The moving die plate 9 is provided with half nuts 29 which engagewith the ring grooves 15 a in the tie bar 15 so that the center thereofcoincides with the axis of the insertion hole 27. The half nuts 29,which are a so-called split nut divided into plural numbers in thecircumferential direction, are configured so as to be opened and closedby a hydraulic cylinder 29 a or the like (see FIG. 2) disposed at rightangles to the axis of the insertion hole 27 (that is, in the radialdirection of the insertion hole 27). Further, on the outside face of themoving die plate 9, a support member 33 for supporting the half nuts 29is provided.

[0030] The support member 33 supports the half nuts 29 to prevent thehalf nuts 29 from separating from the moving die plate 9, for example,when the tie bar engaging with the closed half nuts is pushed in theleft direction in FIG. 1 by an oil pressure applied onto the head sideof the large-diameter ram 13 for the purpose of die opening etc.

[0031] In FIG. 1, reference numeral 19 denotes an injection cylinder,and the two-dot chain line indicates the position of the moving dieplate 9 in the state of die closure.

[0032] In FIG. 2, a detecting section for detecting a relative positionof the moving die plate 9 and the tie bar 15 is enlargedly shown. Also,a part of an oil pressure control system for the die clamping cylinderand a control unit 77 for controlling the positions of the rams 13 and25 through the oil pressure control system.

[0033] This oil pressure control system has oil pressure supply meansincluding oil pressure switching valves 61, 63 and 65, a low hydraulicpower source (not shown), and a high hydraulic power source (not shown).The oil pressure switching valve 61 is a four-way switchingelectromagnetic valve having a block port for supplying a pressure oilto the small-diameter cylinder 31. One port of the oil pressureswitching valve 61 is connected with a high-pressure oil pipe 67, andthe other port thereof is connected with an oil pipe 71. The oil pipe 71is connected to a port 31 a in the fixed die plate 5 communicating withthe small oil chamber 11 e in the small-diameter cylinder 31.

[0034] The oil pressure switching valve 63 is a three-way switchingelectromagnetic valve having a block port for supplying a high-pressureoil (for example, 175 kgf/cm²) from the high-pressure oil pipe 67 or alow-pressure oil (for example, 40 kgf/cm²) from a low-pressure oil pipe69 to the large-diameter cylinder 11 by switching. The oil pressureswitching valve 65 is a three-way switching electromagnetic valve whichapplies a die clamping force and a die opening force to thelarge-diameter ram 13 by switching the oil pressures in the large oilchambers 11 c and 11 d on both sides of the large-diameter ram 13 in thelarge-diameter cylinder 11. An oil pipe 73 is connected to a port 11 acommunicating with the large oil chamber lid on the die clamping side(left-hand side in FIG. 2) of the large-diameter cylinder 11. An oilpipe 75 is connected to a port 11 b communicating with the large oilchamber 11 c on the die opening side (right-hand side in FIG. 2) of thelarge-diameter cylinder 11.

[0035] In the above-described oil pressure control system, when apressure oil is sent to the oil pipes 73 and 75 through the oil pressureswitching valve 63, if an electromagnetic coil Z of the oil pressureswitching valve 65 is excited to operate the oil pressure switchingvalve 65 in the pressure oil discharge direction, in FIG. 2, the oilpressure in the large oil chamber 11 c on the right-hand side of thelarge-diameter ram 13 in the large-diameter cylinder 11 becomes only aslight back pressure (for example, 1 kgf/cm²). Therefore, the pressureoil acts on the left-side face of the large-diameter ram 13, so that adie clamping force, which is a force in the right direction in FIG. 2,is created. On the other hand, when the electromagnetic coil Z of theoil pressure switching valve 65 is non-exited, the oil pressureswitching valve 65 operates in the direction such as to cause a pressureoil to flow, so that the same oil pressure acts on both side faces ofthe large-diameter ram 13 in the large-diameter cylinder 11. At thistime, however, the oil pressure acting area (area subjected to pressure)on the left-hand side of the large-diameter ram 13 is smaller than thaton the right-hand side thereof by the area of the small-diameter ram 25,so that an oil pressure acting force in the die opening direction (theleft direction in FIG. 2) of the large-diameter ram 13 is increased by adifference in oil pressure acting area between both side faces of thelarge-diameter ram 13, whereby a die opening force is created on thelarge-diameter ram 13.

[0036] Next, the sequence of injection molding performed by using theabove-described embodiment will be explained.

[0037] First, a die that serves as a reference (hereinafter referred toas a reference die) is installed to each of the fixed die plate 5 andthe moving die plate 9. Next, the moving die plate 9 is moved by themoving cylinder 17, by which the die 7 is closed. The position at whichthe die is closed is taken as a reference position S₀ of the moving dieplate 9, and the position of the scale 43 detected by the positionsensor 41 of the moving die plate 9 is taken as an initial point 0 forcalculation. The reference position S₀ and the initial point 0 arestored in the control unit 77. Then, in the reference position S₀, theposition of the large-diameter ram 13 is calculated from the distancefrom the attachment face of the die 7 of the moving die plate 9 to theposition of ring groove in the half nuts 29 and the distance from theright end portion of the large-diameter ram 13 in FIG. 2 to the positionof the ring groove 15 a in the tie bar 15 so that the tie bar 15 and thehalf nuts 29 are engaged properly with each other, and the calculatedposition is taken as a reference position T₀ of the large-diameter ram13. The position T₀ is stored in the control unit 77 as the initialpoint 0 for the calculation of the tie bar position detecting positionsensor 55.

[0038] At the time of operation of the injection molding machine, first,as shown in FIG. 2, the oil pressure switching valve 63 is operated to aposition at which a pressure oil does not flow (hereinafter referred toas a block position), and the oil pressure switching valve 65 isoperated in the direction in which a pressure oil flows by making theelectromagnetic coil Z non-excited. Thereby, the difference in oilpressure between the large oil chambers 11 c and 11 d on both sides ofthe large-diameter ram 13 in the large-diameter cylinder 11 is madezero. In this state, an electromagnetic coil X₂ of the oil pressureswitching valve 61 is excited, by which a high-pressure oil is sent tothe port 31 a. Thereby, the small-diameter ram 25 in the small-diametercylinder 31 is pushed by the high-pressure oil, so that thesmall-diameter ram 25 is retreated (moved in the right direction in FIG.2) together with the large-diameter ram 13. Next, when thelarge-diameter ram 13 arrives at a reference position T1, anelectromagnetic coil X₁ is excited to make the oil pressure switchingvalve 61 in the block position, and the large-diameter ram 13 is stoppedat the reference position T₁. In this state, the moving die plate 9 ismoved to close the die.

[0039] At this time, the large-diameter ram 13 is located at a positionat which the half nuts 29 just engage with the ring grooves 15 a in thetie bar 15. Therefore, by moving the half nuts 29 toward the axisthereof by the hydraulic cylinder 29 a, the half nuts 29 can be engagedwith the ring grooves 15 a in the tie bar 15 (see FIG. 2).

[0040] At this time, since a small gap exists between the ring groovesin the half nuts 29 and the ring grooves 15 a in the tie bar 15, afterthe half nuts 29 are engaged with the ring grooves 15 a, theelectromagnetic coil X₂ of the oil pressure switching valve 61 isexcited to send a high-pressure oil to the port 31 a. Thereby, thesmall-diameter ram 25 is pushed by the oil pressure in thesmall-diameter cylinder 31, by which the tie bar 15 is pulled in theright direction. Therefore, the engagement gap between the ring grooves15 a in the tie bar 15 and the half nuts 29 is eliminated, and also agap between the half nuts 29 and the moving die plate 9 and other gapsare eliminated.

[0041] Thus, by supplying a pressure oil into the small oil chamber 11 ein the small-diameter cylinder 31, the engagement gap between the tiebar 15 and the half nuts 29 is eliminated, by which the aforementionedgap can be eliminated by using a smaller quantity of hydraulic oilwithout the use of a large-diameter die clamping cylinder. Moreover, thetie bar 15 can be coupled to the half nuts 29 without a gap in a shortperiod of time, so that a mechanical shock occurring at the time of dieclamping can be restrained.

[0042] The operation to perform die clamping is as described below.

[0043] First, the electromagnetic coil Z of the oil pressure switchingvalve 65 is excited to operate the oil pressure switching valve 65 inthe direction such as to stop a pressure oil. Next, an electromagneticcoil Y₂ of the oil pressure switching valve 63 is excited to operate theoil pressure switching valve 63 so as to cause a high-pressure oil toflow, by which the high-pressure oil is sent to the large oil chamberlid on the tie bar side of the large-diameter ram 13 in thelarge-diameter cylinder 11. At the same time, the electromagnetic coilX₂ of the oil pressure switching valve 61 is excited to operate the oilpressure switching valve 61 so as to cause the high-pressure oil toflow, by which the high-pressure oil is sent to the small-diametercylinder 31. Thereby, the tie bar 15 is pulled toward the fixed dieplate 5 with a high force, so that the die clamping operation isperformed.

[0044] After the die clamping operation is finished, a product is takenout as described below.

[0045] First, the electromagnetic coil X₁ of the oil pressure switchingvalve 61 is excited to make the internal pressure of the small-diametercylinder 31 zero (atmospheric pressure), and an electromagnetic coil Y₁of the oil pressure switching valve 63 is excited to operate the oilpressure switching valve 63 so as to communicate with the low-pressureoil pipe 69. At the same time, the electromagnetic coil Z of the oilpressure switching valve 65 is made non-exited to operate the oilpressure switching valve 65 so as to cause the pressure oil to flow.Thereby, the pressures on both sides of the large-diameter cylinder 11is made equal. As a result, by the difference in oil pressure actingarea of the large-diameter ram 13 between the large oil chamber 11 c andthe large oil chamber 11 d, the large-diameter ram 13 is moved in thedie opening direction (the left direction in FIG. 2). Thereby, the diecan be opened slightly in a state in which the tie bar 15 is fixed bythe half nuts 29. Thereafter, the half nuts 29 are opened to release theconnection between the half nuts 29 and the tie bar 15, and the movingdie plate 9 is moved in the direction such as to separate from the fixeddie plate 5 by the moving cylinder 17, by which the product is taken outby using a take-out robot or the like (not shown).

[0046] Before the next die clamping operation is started, the followingoperation is performed.

[0047] First, the oil pressure switching valve 63 is made in the blockposition (position at which the pressure oil does not flow). Next, theoil switching valve 65 is operated so as to cause the pressure oil toflow to open a passage between the large oil chamber 11 c and the largeoil chamber lid on both sides of the large-diameter ram 13, by which thedifference in pressure between the large oil chamber 11 c and the largeoil chamber 11 d is made zero. Furthermore, the electromagnetic coil X₂of the oil pressure switching valve 61 is excited to switch over the oilpressure switching valve 61 to the direction such as to send thehigh-pressure oil to the port 31 a. As a result, the small-diameter ram25 is pushed in the right direction in FIG. 2 by the pressure oil in thesmall oil chamber 11 e in the small-diameter cylinder 31. Thereafter,when the large-diameter ram 13 is located at the reference position T₁,the electromagnetic coil X₁ of the oil pressure switching valve 61 isexcited to make the interior of the small-diameter cylinder 31 atatmospheric pressure. Subsequently, by the same operation as describedabove, the next die clamping operation can be performed. The flow ofwork in such a die clamping process is shown in a block diagram of FIG.3.

[0048] In FIG. 2, the position SO indicated by the two-dot chain line isa position of the moving die plate 9 for fastening the half nuts 29 whenthe reference dies are installed to the die plates 5 and 9. At thistime, the moving die plate 9 is in the state in which the movement fordie closure is finished and the die clamping operation is not yetstarted. On the other hand, the large-diameter ram 13 is located at theposition T₁, which position is indicated by the two-dot chain linedesignating the face on the side opposite to the small-diameter ram 25(on the right side in FIG. 2).

[0049] Next, a case where the die is replaced with another die with athickness different from that of the reference die (hereinafter referredto as a replacement die) will be explained.

[0050] In this case, since there is a possibility that the engagement ofthe ring grooves 15 a in the tie bar 15 with the ring grooves in thehalf nuts 29 shifts, it is necessary to correct the shift of engagementpitch of the ring grooves 15 a in the tie bar 15 to attain properengagement. The position of the moving die plate 9 at this time is takenas S, and the position of the large-diameter ram 13 in thelarge-diameter cylinder 11 is taken as T₂. These positions S and T₂ areindicated by solid lines in FIG. 2.

[0051] In order to correct the shift of engagement pitch of the ringgrooves 15 a in the tie bar 15, a dimensional difference m in thicknessbetween the reference die and the replacement die is first determined.This dimensional difference m is determined from the actually measuredvalue of replacement die. Alternatively, it is determined as follows: adifference in position when the reference die and the replacement dieare installed to the moving die plate 9 that is in the die closure stateis detected on the scale 43 by the position sensor 41, and the detectionresult is calculated by the control unit 77. The dimensional differencem is divided by a pitch c of the ring grooves 15 a, and a residualsmaller than indivisible c is taken out. This residual is used as acorrection value, and is stored in the control unit 77 as the positionof the large-diameter ram 13.

[0052] This process is expressed by a formula as described below.

e=e ₀ +m−n×c (where m−n×c<c)

[0053] where,

[0054] e: a gap between the large-diameter ram 13 and the left-side wallface of the large-diameter cylinder 11 produced when the replacement dieis installed to the moving die plate 9 (at this time, the right-side endof the large-diameter ram 13 is located at the position T₂)

[0055] e₀: a gap between the large-diameter ram 13 and the left-sidewall face of the large-diameter cylinder 11 produced when the referencedie is installed to the moving die plate 9 (at this time, the right-sideend of the large-diameter ram 13 is located at the position T₁) n, n′:integer

[0056] In FIG. 2, T₀ indicates a position of limit end to which thelarge-diameter ram 13 can be moved to the left side in thelarge-diameter cylinder 11.

[0057] When the moving die plate 9 moves from the reference position S₀through the distance m, the left-side gape of the large-diameter ram 13in the large-diameter cylinder 11 can be determined by theaforementioned formula. This calculation for determining e is performedby the control unit 77. The value e thus determined is stored in thecontrol unit 77, and before the next die clamping operation is started,the tie bar 5 is moved in the right direction in FIG. 2 through adistance of e. When the injection molding machine is operated, the valuedetected by the scale 57 is compared with this value e. When thesevalues become equal to each other, the large-diameter ram 13 is stopped.

[0058] A stroke of ram in the large-diameter cylinder 11 that is atleast twice the pitch c of the ring grooves 15 a in the tie bar 15suffices because the operation can be performed. Also, in the method foradjustment to die thickness using the die clamping apparatus of thisembodiment, the large-diameter ram 13 has only to be returned slowly tothe position T₂ (or T₁) during the time from when the die opening byusing the large-diameter cylinder 11 is finished to when the next dieclamping operation is started, so that the supply of pressure oil to thesmall oil chamber lie in the small-diameter cylinder 31 can be effectedby using a low-capacity hydraulic power source.

[0059] Also, when the die clamping apparatus of this embodiment isoperated, the large-diameter ram 13 has only to be returned to anarbitrary corrected position T (not shown) before the die clampingoperation.

[0060] Further, in this embodiment, the opening position of the movingdie plate 9 (a clearance between the moving die plate 9 and the fixeddie plate 5) can be set by combining the position sensor 41 fordetecting the position in the travel direction of the moving die plate9, the scale 43, and the control unit 77.

[0061] Further, although the tie bar 15 is fastened to the half nuts 29in the state of die closure in this embodiment, even in the state inwhich the moving die 7 and the fixed die 3 are open before the die isclosed, if (e−m) is an integral multiple (n′) of the ring groove pitch cas expressed by a formula of (e−m=n×c+n′×c), the tie bar 15 can befastened to the half nuts 29. In this case, the distance between themoving die 7 and the fixed die 3 is n′×c.

[0062] As described above, in this embodiment, the fixed die plate 5 isprovided with the two-stage die clamping cylinder consisting of thelarge-diameter cylinder 11 and the small-diameter cylinder 31 so thatthe two-stage ram consisting of the large-diameter ram 13 and thesmall-diameter ram 25 is slidingly moved in the die clamping cylinder,and the proximal end portion of the tie bar 15 is connected directly tothe face on the moving die plate side of the small-diameter ram 25. Onthe other hand, the moving die plate 9 is provided with the insertionhole 27 for the tie bar 15, and the support member 33 for supporting thehalf nuts 29 is provided. This positional relationship may be changed asdescribed below. The moving die plate 9 is provided with a two-stage dieclamping cylinder consisting of a large-diameter cylinder and asmall-diameter cylinder so that a two-stage ram consisting of alarge-diameter ram and a small-diameter ram is slidingly moved in thedie clamping cylinder, and the proximal end portion of the tie bar isconnected directly to the face on the fixed die plate side of thesmall-diameter ram. On the other hand, the fixed die plate 5 is providedwith an insertion hole for the tie bar, and a support member forsupporting half nuts is provided.

INDUSTRIAL APPLICABILITY

[0063] As described above, according to the present invention, thenumber of construction parts is small as compared with the constructionof the conventional die clamping apparatus, so that the die clampingapparatus in accordance with the present invention can be manufacturedat a low cost. Moreover, a large operation space can be provided on theside opposite to the die attachment face of the fixed die plate, so thatthe operability of the apparatus can be increased.

[0064] Further, the relative stop position of the moving die plate withrespect to the tie bar can be selected freely, so that the apparatus canaccommodate dies with various die thicknesses. Moreover, since the tiebar can be engaged with the ring grooves (or thread groove) in the halfnuts at any position, the apparatus achieves an effect that it can beused easily for injection compressing or cloth-laid molding.

[0065] Still further, in the present invention, even if a small gapexists between the ring grooves of half nuts and the ring grooves of tiebar after they are engaged with each other, the oil pressure switchingvalve is switched over again in the state in which the large-diametercylinder is under no pressure to send a high-pressure oil to thesmall-diameter cylinder, and the tie bar is pulled by using thesmall-diameter ram in the small-diameter cylinder, by which the gap inthe aforementioned engagement is eliminated. Therefore, the operationfor eliminating the gap can be performed with a small quantity ofpressure oil as compared with the case where the gap is eliminated bypulling the tie bar using a large-diameter die clamping cylinder. Thus,the tie bar can be connected to the half nuts without a gap in a shortperiod of time. Moreover, since a gap between the half nuts and themoving die plate and other gaps can be eliminated in a short period oftime, useless time for eliminating a gap between the ring grooves in thetie bar and the engagement teeth of the half nuts caused by theadjustment to die thickness can be reduced, and further a mechanicalshock occurring at the time of die clamping can be restrained.

1. A die clamping apparatus comprising a fixed die plate for holding afixed die; a moving die plate for holding a moving die; displacing meansfor advancing and retreating said moving die plate with respect to saidfixed die plate; and die clamping means, which is incorporated in eitherone of said fixed die plate and said moving die plate, for coupling andpressing said moving die plate to said fixed die plate via a tie bar,wherein said die clamping means is provided with a die clamping cylindercomprising of two stages of a large-diameter cylinder and asmall-diameter cylinder, and a ram formed into two stages of alarge-diameter ram and a small-diameter ram; said large-diameter ram isinserted in said large-diameter cylinder, and said small-diameter ram isinserted in said small-diameter cylinder; the proximal end portion ofsaid tie bar is connected directly to said small-diameter ram; a largeoil chamber is provided on each side of said large-diameter ram, and asmall oil chamber is provided on the tie bar connection side of saidsmall-diameter ram; and oil pressure supply means is provided whichappropriately supplies an oil pressure to said large oil chamber andsaid small oil chamber in said die clamping cylinder to appropriatelymove said tie bar.
 2. The die clamping apparatus according to claim 1,wherein a plurality of ring grooves with an equal pitch are formed inthe distal end portion of said tie bar; half nuts, which engage with thering grooves in said tie bar, for regulating the movement of said movingdie plate are disposed on said fixed die plate or said moving die platewhich faces the die plate incorporating said die clamping means; andfurther there are provided tie bar position detecting means fordetecting the position of said tie bar, moving die plate positiondetecting means for detecting the position of said moving die plate, anda control unit for supplying a pressure oil to said small oil chamber byusing said oil pressure supply means based on a detection signal sentfrom said tie bar position detecting means and said moving die plateposition detecting means and controlling the position of said ram sothat said half nuts and said tie bar engage with each other. 3.(Amended) The die clamping apparatus according to claim 1 or 2, whereinsaid oil pressure supply means comprises a first oil pressure switchingvalve for supplying a high-pressure oil to said small oil chamber orstopping the supply, a second oil pressure switching valve forselectively supplying either a high-pressure oil or a low-pressure oilto said large oil chambers on both sides of said large-diameter ram orstopping the supply, and a third oil pressure switching valve forsupplying the pressure oil sent from said second oil pressure switchingvalve to the large oil chamber located on the side opposite to thesmall-diameter ram of said large-diameter ram or stopping the supply. 4.A die clamping method in which using the die clamping apparatus asdescribed in claim 1, a pressure oil is supplied to said small oilchamber to move said tie bar, and after an engagement gap between saidhalf nuts and said tie bar is eliminated, a pressure oil is supplied tosaid small oil chamber and said large oil chamber on the small ram sideto create a die clamping force.
 5. A method for adjustment to diethickness, in which when adjustment to die thickness is made using thedie clamping apparatus as described in claim 1, said ram is located at aposition of limit end to which said ram can be moved, and after thepressures in said large oil chambers on both sides of saidlarge-diameter ram are decreased, a pressure oil is supplied to saidsmall oil chamber to move said ram in the die clamping direction, bywhich said tie bar is located at a position at which the ring grooves insaid tie bar engage properly with the ring grooves in said half nuts.